a. Field of the Invention
The present invention relates to a ring trip circuit for a subscriber telephone, by which an on-hook state or an off-hook state of the telephone is detected. More particularly, it relates to an improved ring trip circuit which has a reduced size and can be formed by a large scale integrated circuit.
b. Description of the Related Art
Ring trip circuits are provided in a telephone exchanger (or switching board) for detecting an on-hook state or off-hook state of subscriber telephones. Each ring trip circuit is connected to a subscriber line, i.e., a ring trip circuit must be provided for each subscriber telephone. Since the telephone exchanger processes a large number of subscriber lines, for example, over 20,000 subscriber lines, a large number of ring trip circuits are provided in the telephone exchanger.
The ring trip circuit of the prior art are bulky and are not suitable to be formed by a large scale integrated circuit (LSI). Also, the ring trip circuits of the prior art are expensive.
The prior art ring trip circuit will be described in more detail later with reference to the drawings.